Life in a cold environment - Antarctica: Global Science from a Frozen Continent - page 183

presumably with appropriate physiological pre-adaptations, then underwent a
dramatic evolutionary radiation generating many key elements of the present-day
benthic marine fauna. Evolutionary radiations have taken place in Antarctica in
isolation over long periods of time, leading to typically high levels of species
restricted to the area (60

90%). This is the case for sponges, bryozoans, polychaetes,
pycnogonids, ascidians, anemones, peracarid crustaceans and some bivalves. The
speciation processes in krill and notothenioid

-

fish have received particular research
attention, along with peracarid crustaceans and pycnogonids.
The Southern Ocean fish fauna has relatively low diversity, but it shows two

striking radiations: the nototheniids on the continental shelf and the liparids on the
continental slope. Therefore, the fish groups with the highest numbers of species

today are the notothenioids, liparids and zoarcids, which between them account for
88% of species diversity; coincidentally, 88% of Antarctic fish species are also

endemic. The level of dominance and the radiation of nototheniid fishes are unique

and exhibit many parallels with species flocks of freshwater fishes in some ancient

lakes. The notothenioid diversi cation was accompanied by considerable

morphological and ecological changes. For instance, the most advanced adaptation
to ice as a microhabitat for Antarctic fish has been observed for Pagothenia

borchgrevinki , which has been reported to cling to the subsurface of the marginal ice
shelf of the Drescher Inlet.
The isopod family Serolidae is likely to have originated between 90 and 55million
years ago. This estimate, based on linkages between geological processes and
biogeographical patterns, roughly coincides with estimates for evolutionary
radiations in some amphipod taxa. The inferred age of the last common ancestor of
iphimediid amphipod species is 34.4million years, approximately coinciding with
the opening of the Drake Passage separating the Antarctic Peninsula from South
America. The timeline of speciation, as well as their restriction to Antarctic waters,
are both consistent with the view that the epimeriid and potentially also the
iphimediid species evolved in the Southern Ocean when it was already cold and
isolated from other fragments of Gondwana.
The Southern Ocean decapod fauna comprises only 98 benthic species in the
entire region, with 92 species occurring on the continental shelves and around the
sub-Antarctic islands and 6 species in the deep sea. The general global restriction of
the Brachyura to shallow-water zones may underlie the absence of this group on
Antarctic continental shelves, assuming successive eliminations of shallow water
fauna by ice sheet expansion, scouring and anchor ice during glacial maxima. The
striking Tertiary extinction and continued absence of the creeping decapods (e.g.
crabs, lobsters) has been suggested as being due to physiological constraints of
magnesium regulation in the cold water. The first recent records of anomuran and

brachyuran larvae were obtained from the Brans eld Strait off the Antarctic